Parasites may be characterized as ectoparasites—including ticks, fleas, leeches, and lice—which live on the body surface of the host and do not themselves commonly cause disease in the host; or endoparasites, which may be either intercellular (inhabiting spaces in the host’s body) or intracellular (inhabiting cells in the host’s body). Intracellular parasites—such as bacteria or viruses—often rely on a third organism, known as the carrier, or vector, to transmit them to the host. Malaria, which is caused by a protozoan of the genus Plasmodium transmitted to humans by the bite of an anopheline mosquito, is an example of this interaction. The plant ailment known as Dutch elm disease (caused by the fungusCeratocystis ulmi) can be spread by the European elm bark beetle.

The life cycle of the hard tick Ixodes scapularis, a carrier of the bacterium that …

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Life cycle of a malaria parasite.

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A different form of parasitism called brood parasitism is practiced by most species of cuckoos and all cowbirds. Those birds do not build nests of their own but deposit their eggs in the nests of other species and abandon them there, with the hope that adult birds of other species will raise the abandoned young as their own. The cowbird’s parasitism does not necessarily harm the host or the host’s brood; however, the cuckoo may remove one or more host eggs to reduce the suspicion surrounding the presence of its egg, and the young cuckoo may heave the host’s eggs and nestlings from the nest.

The European cuckoo (Cuculus canorus) has coevolved with several other bird species, …

Another form of parasitism, such as that practiced by some ants on ants of other species, is known as social parasitism. (Social parasitism is a condition where a parasitizing ant species depends upon the labour provided by a host ant species within the context of a mixed-species colony.) Parasites may also become parasitized; such a relationship, known as hyperparasitism, may be exemplified by a protozoan (the hyperparasite) living in the digestive tract of a flea living on a dog.

Sexual parasitism, which is actually a type of specialized reproduction, is most commonly associated with deep-sea anglerfish, where it occurs in more than 20 species. In these fish, males are much smaller than females. (In the case of the northern seadevil, or deep-sea angler, Ceratias holboelli, females may be more than 60 times the size of males.) Females possess a luring apparatus to entice prey, but males do not. However, males possess the visual and olfactory acuity to locate females so that they might obtain food. Males attach themselves to females with their jaws, and in some cases the tissues and circulatory systems between the sexes are joined. Thereafter, the male serves as a sperm-producing organ on the female, since transformation makes him completely dependent upon her.

Other forms of sexual parasitism also exist, including those in which the genetic material from one parent is discarded by the other parent despite the effort made by the other parent to produce and deliver it. For example, young resulting from the pairing of sailfin mollies (Poecilia latipinna) and Atlantic mollies (P. mexicana) are females that can produce only clones of themselves. They need sperm from males of either of the two species to start the process; however, since all offspring are clones of their mother, no male DNA is passed on.

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The speciation process may also be initiated by changes in just one or a few gene loci when these alterations result in a change of ecological niche or, in the case of parasites, a change of host. Many parasites use their host as a place for courtship and mating, so organisms with two different host preferences may become reproductively isolated. If the hybrids show poor fitness because they...

...uniflora; Ericaceae) that use connections with mycorrhizal fungi (fungi that form an association with the roots of certain plants) to obtain carbohydrates from dead organic material or parasites that develop specialized roots (haustoria), which penetrate the host plant and absorb food and other materials (e.g., the dodder [Cuscuta salina; Convolvulaceae]).

Diseases caused by fungi and parasites are relatively uncommon in developed countries. Fungal infections, also known as mycotic infections, may affect the skin surfaces or the internal organs of the body. The superficial mycotic infections are generally not serious and include such well-known disorders as athlete’s foot (tinea pedis), caused by the dermatophyte Trichophyton. Deep mycotic...

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A parasite is an organism, or living thing, that lives on or inside another organism. It depends on the other organism for food and other things that it needs to live. The parasite’s victim is called its host. The host is usually much larger than the parasite.

An organism that lives on or within another organism, called the host, and that gains its sustenance from the host organism is known as a parasite. Parasites occur among all the major groups of living things. There are parasitic fishes-for example, the lamprey, which attaches itself to other fishes and sucks their body fluids. There are many parasitic arthropods, including fleas, lice, biting flies, and mosquitoes. (See also insect; invertebrates.)